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How our histology lab became a 'clean' industry.

How our histology lab became a 'clean' industry

This hospital's discovery, analysis, and elimination of xylene pollution comprise an interesting case history in problem solving.

In 1985, our 562-bed hospital started testing its wastewater effluent as required by a county ordinance. We were stunned to learn that xylene in the effluent far exceeded the acceptable level. A review of potential sources traced the excess back to the histology laboratory. How we analyzed and solved our pollution problem is a story you may find interesting. In fact, you may face just such a problem in the near future.

Over the last two decades, many government bodies have adopted new regulations in response to growing concern over environmental pollution. In our state and particularly our community, the concern is acute because water is such a precious commodity. Tucson is probably the largest city in the United States that is completely dependent upon well water--we have no reservoirs.

Laboratorians themselves have become increasingly aware that safety procedures and safety-engineered laboratory design are needed to guard against accidental chemical exposures and unplanned discharges. Reflecting this consciousness, College of American Pathologists' inspections have focused more and more on storage, labeling, use, and disposal of hazardous materials. As examples, azide and other carcinogenic reagents are no longer found in use, and formaldehyde is largely restricted to semi-closed systems and sites with adequate ventilation.

Many chemistry and hematology test methodologies have gone to microprocessing with very small amounts of reagent. Histology, however, continues to use large quantities of organic solvents and remains a major producer of organic solvent effluent, though the move toward microwave fixation may curtail some form aldehyde use in the near future.

Our hospital has always been a leader in promoting a healthy environment. For example, it was one of the first smoke-free hospitals in the region, That's why we were so surprised when our effluent tests yielded results well above the acceptable levels for xylene and its congeners (10 Kgm/L), as the August 1986 concentrations in Figure I demonstrate.

After checking our institution's hazardous chemical inventory, we quickly located the only possible source for bulk amounts of xylene: histology (specifically the automatic slide stainer). Far from settling the matter, that was just the beginning of several months of detective work.

"How are we disposing of the used xylene?" we asked. "By siphoning it down the drain," histology replied. So we immediately instituted a recycling program, which sharply reduced xylene levels. Yet our values still stood significantly higher than the acceptable limits. Where was the problem now?

Armed with drawings of the hospital's plumbing system, we inspected the wastewater's flow downstream from the histology laboratory. It entered an acid neutralization pit. Testing there of both the sludge and aqueous components indicated extremely high levels of the offending compounds (Figure II). Since the laboratory had already stopped all xylene dumping, we reasoned that a residual buildup of xylene products gradually leaching out of the pit caused the continued contamination.

This led us to shut the system down and give the pit a thorough cleanout and scrubbing. Confident that the problem was finally solved, we hooked up all the lines and reopened the pit. The next set of effluent values was better-but still not in compliance.

At this point, we began to appreciate just how tough the county's ordinance was. We realized that conforming to the standard would require a major systems change in the way we disposed of xylene byproducts.

First, we tested wastewater drawn directly from the automatic slide stainer and assured ourselves that it was the only source of the effluent solvents. Then we considered ways to capture the xylene before it entered the wastewater lines.

One possibility was containerized storage and disposal of the autostainer's effluent as bulk hazardous waste. We soon realized this was neither fiscally nor physically feasible. The 300 to 400 liters generated daily would fill a pickup truck.

Since we were dealing with very small xylene concentrations in the wastewater (20,000 Kgm/L), filter separation of the offending compounds was a logical alternative (Figure III). The Granular Activated Charcoal (GAC) filtration system is widely employed for treatment of both drinking water and wastewater, and we felt certain we could modify the technique to solve our problem.

The basic principle of GAC action rests on the combination of physical adsorption (resulting from weak van der Waal forces) and stronger chemical bonding. The "activation" of GAC refers to a method--either thermal or chemical--mf increasing the already vast porous surface area of the charcoal granules. GAC used for water treatment often has more than 500 to 1,500 square meters of surface area per gram.

The hospital's engineering and maintenance staff planned and detailed an appropriate GAC filtration system, and we took the proposal to a major GAC manufacturer. This firm had a product that met our specifications, but even better, it offered a modular GAC filtration canister that was economical and adaptable to our needs. Hospital engineering and maintenance personnel were willing to fabricate, install, test, rework, and maintain the new system until we were all satisfied with its performance.

Figure IV shows the system our staff designed: 1) a holding tank with level control, 2) a pump to move the effluent through the lines, and 3) a GAC filter canister. The autostainer effluent drains via gravity into the holding tank. When the tank's contents reach a preset level, the elective level controller switches on the pump, which moves the effluent through a 1/2-inch stainless steel pipe to the GAC filter module.

At our flow rates of 750 to 1,200 cc per minute, the effluent interfaces with the GAC in the canister for about one hour. A separate pipe continuously discharges the treated autostainer effluent into an ordinary laboratory sink drain line. As Figure V indicates, the treated effluent exhibits substantially reduced levels for all chemicals, but most of all for those we originally found to be noncompliant. Most important, we were able to achieve consistent compliance.

There were a few bugs still to be worked out. First, we had to sit down with the histology staff to design and install the system and its controls so that it would fit in the existing layout and be easily accessible. Then we had to do something about the odor of rotten eggs that wafted through the laboratory and occasionally throughout the hospital.

The smell rose out of the sink that drained the GAC-treated effluent. Although the laboratory's air monitors registered barely detectable levels of hydrogen sulfide gas, staff members could tell the gas was present-human perception threshold levels are less than 0.5 parts per billion. We knew that the effluent's acidic nature (a pHof 5.8 to 6.3) coupled with the lengthy contact time in the GAC canister was sufficient to liberate hydrogen sulfide gas from the Harris Alum Hematoxylin staining solution, which contains 47.4 mg/L of aluminum ammonium sulfate.

The total sulfide levels of the hospital's sewer effluent were well below the levels permitted by the county ordinance. The histology sink was causing all the problems. We took care of this by using a P-trap device to divert the GAC-treated effluent before it entered the laboratory sink drain. The P-trap creates a watertight seal in the discharge system, preventing any back-flushing of the small amounts of hydrogen sulfide gas during the effluent's entry into the drainage system beyond the laboratory sink.

Small GAC canisters were installed outside on the vents connected to the lab's drain system. Switching to a different type of carbon in these rooftop canisters helped. This product neutralizes the acid coming off the airstream and raises the pH so that the carbon's adsorption can deal effectively with the odor.

Histology's GAC treatment system has been operating for a year, and our effluent readings continue to be extremely low. We change the carbon filter in the canister system every six months. Effluent testing takes place every two or three months.

All of this costs about $1,000 a year--$600 for supplies and $400 for labor. There's no problem justifying the expense. Consider that the hospital could face a daily fine of $25,000 per violation of the county ordinance. And on a more positive note, consider that we have turned the histology laboratory into a "clean" industry.
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Title Annotation:discovery, analysis & elimination of xylene pollution
Author:Grushka, Mark J.; Spark, Ronald P.
Publication:Medical Laboratory Observer
Date:Jun 1, 1988
Words:1388
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